In a medium bowl, toss the shrimp with the lemongrass, fish sauce, honey, garlic, and ginger. Season with a pinch of salt. Set aside.
Line a plate with paper towels. In a large cast-iron skillet, heat the oil over medium-high heat until shimmering, about 2 minutes. Add the shallots and cook, stirring constantly, until golden, about 3 minutes. Transfer shallots to the plate using a slotted spoon. Leave oil in pan.
Add the marinated shrimp to the skillet in a single layer. Cook over medium-high heat, until golden on both sides, 2 to 3 minutes (turning halfway through). Transfer to a plate.
In a medium bowl, whisk together the grapefruit juice, lime juice, honey, and fish sauce.
In a large bowl, gently toss the grapefruit with the fennel, 1/3 of the fried shallots, and the herbs.
Divide rice noodles among 4 plates. Top each plate with 1/4 of the grapefruit salad and shrimp, and drizzle generously with dressing. Garnish with remaining shallots and cashews, and serve immediately.
The anise flavor of the fennel plays off the grapefruit's tartness.Recipe by Laura Rege.